The use of optical cable for carrying signals from a seismic tool greatly increases the amount of signals that can be sent. This gives new possibilities in the seismic understanding of reflected signals and new interpretation in order to understand subterranean events. The limitation is the actual number and type of seismic tools that are connected and operated on the electric or fibre optic cable.
New understanding of additional signals on the fibre optic cable caused by seismic reflection signals gives a greater possibility of collecting large amounts of additional information from the fibre optic cable in itself combined with existing seismic tools/gamma ray connected to the electric cable.
By understanding vibration signals on a production well string equipped with a plurality of levels of instruments, caused by inflow of water/gas/oil/sand vibrations, it will be possible to detect what is inside the string and where, as well as monitoring the condition of the well components.
Abandoning working oil/gas wells can be a complicated business in dangerous surroundings where there is a high risk of pollution if leakages occur. A system as described according to the invention can secure such wells after they have been abandoned and will be an important contribution to avoiding undesirable incidents. These incidents include both leakages from the well itself and leakages from geostructures around the well.
Closing down oil/gas fields, in particular in environmentally dangerous areas, will in the future lead to requirements that leakage from shut-down fields should be monitored. The system according to the present invention will be especially suitable in the final phase and after the shutting down of wells and fields. It will give information about how the field develops after shutdown. Gas that may penetrate into the well in the final phase could cause high pressure against the seabed/surface, and gas could penetrate into the geoformation and cause pollution and, at the worst, blowout. The system's microseismic detection of seismic events in the reservoir can give information before such leakages break out and thus avert disasters.
A seismic instrument is able to collect from the earth's surface or the seabed enormous amounts of signals from events down in a reservoir, in contact with the seabed, in close contact with the seabed, in contact with the earth's surface or in contact with the well wall in a well. The problems of bringing the signals up to the surface have previously been limited by the capacity of the cable with feedthrough through the wellhead etc. A marked increase in transmitted signal amount can be obtained by using a fibre optic cable.
Geophones have traditionally sent signals via electric cable as three-component measurements. However, more modern geophones have fibre optic signals that require fibre optic cables and are also able to collect three-component measurements.
Fibre optic cables are also able to collect seismic signals via fibre optic signals as one-component measurements. By means of processing equipment, all these signals can be combined to improve the seismics qualitatively and quantitatively.
A fibre optic cable is able to respond to sound signals, vibrations, temperature etc. Light signals that are sent down in the fibre by means of the fibre optic transmitter-receiver (hereafter called “fibre optic gun” or simply “gun”) are reflected or returned by a signal that is analysed. The signals can also be reflected or in some other way returned from one or more given points on the cable, e.g., corresponding to one-meter intervals along the cable. The cable responds to changes such as sound, vibrations, temperature etc. The collected data from the cable point can be compared with the signals from the instrument, and information of both a qualitative and quantitative nature can thus be drawn out from this combination.
A seismic instrument comprising at least one electrical or electromechanical multicomponent seismic sensor, such as a geophone, can take in signals from an event in three directional components (x, y, z), i.e., that the event can be detected with respect to where it comes from. With reflections from a specific geological structure and collection using several such seismic instruments, it can be determined where this structure is relative to the location of the seismic instruments. The signal that creates reflected signals is traditionally an acoustic signal from a seismic source, as, for example, a dynamite shot, and airgun shot or an ignition signal from an electrical source. Similarly, it is possible to locate an event in a reservoir from an earthquake, or from smaller movements that may correspond to a minor earthquake that could be caused through gas leakage, or by gas or by water front movements caused by gas, water and movements of the oil front in a reservoir.
The object of the invention is to find the condition of a well or the well elements with greater precision and/or with a lower use of resources than is previously known.
The present invention provides a technical solution for obtaining and/or processing seismic signals or acoustic signals emitted from a well formation or well installation.
The present invention provides apparatus and/or methods for seismic signal capture, with combinations in one and the same well seismic cable of seismic multicomponent instruments in an electrical portion and distributed fibre optic seismic sensors in a fibre optic portion, and with combinations of the signals detected by seismic multicomponent instruments in the electrical portion of the cable and the signals detected by the fibre optic portion of the cable.
The signals are processed and interpreted such that the events from the reservoir that are detected using multicomponent seismic sensors are amplified and expanded by the signals from a fibre optic seismic sensor cable, both qualitatively and quantitatively (in terms of area).
In accordance with an aspect of the invention, the condition of the well and/or well elements is found by analysing the mechanical movements of the well tubing caused by production vibrations picked up by the signals that the seismic instruments and fibre cable send out.
In accordance with another aspect of the invention, there is provided condition monitoring of the well, inflow data such as amount and composition in the different zones by analyses using a plurality of instruments at different levels in the well and signals from the fibre optic cable actually in the well.
In accordance with an aspect of the invention, it is envisaged that measurements in a well are carried out easily by following a slug of gas on its journey up through a well tubing and, through analysis of the vibration signals, seeing how fast this slug moves so as to determine the volume flow in the well. Through such vibration analyses it is found where gas, water and oil enter the well, and how much, and optionally also what composition is found in the different zones, for example, by utilising the fact that water, gas, oil and sand have different density. Deflection of the well tubing occurs in the perforation zone, and in accordance with an aspect of the invention, it comprises detection and analysis of the well tubing vibrations that may be different depending on the support and deflection of the well tubing and on volume flow and composition.
In accordance with an aspect of the invention, it comprises registering the well tubing vibrations at a plurality of levels, and an analysis of where these vibrations come from, which gives information about volume flow, composition and zone in which the volume flow enters the well tubing.
The inventive system is intended to be realised by using different seismic detection means as, for example, means that are described in US 2012/0162639 A1 in the name of Silixia, published on 28 Jun. 2012. For example, in realising the present invention, use is made of technology as disclosed in paragraph 0017, which describes the use of acoustic sensors.
According to the present invention, a new solution is provided involving a combination of electric signals, mechanical vibration signals and fibre optic signals.
According to a further aspect of the present invention, a solution is provided involving the utilisation of signals that in previously known solutions are removed from the signals for a seismic profile in order to make the profile clearer, i.e., the signals which in previously known solutions for acquiring a seismic profile are the so-called noise signals. In accordance with the invention, the noise signals' content of major mechanical vibration signals is exploited, and used to detail seismic events or to monitor the condition of the well and well elements or for production volume flow measurements and composition measurements.
In the said Silixia document, paragraph 0088 describes a chain of sensors across the perforation zone (drainage zone), but it is well known that cables across the actual perforation zone are undesirable in a well, in particular because of maintenance. The present invention comprises, however, a solution with sensors adapted for mounting above the perforation zone, which acquire signals that are descriptive for inflow and composition in the perforation zone, and which can comprise processing that obtains measurement of inflow and composition in the perforation zone, without installing cable across the actual perforation zone. The present invention advantageously uses detection of mechanical vibrations to analyse this.